The EPR investigation of the age dependence of transferrin (Tr) content and Tr saturation in the serum of 7 groups of Fisher 34 rats (each group containing 5-6 animals) ranging from 2-30 months has been done. The results show that both Tr content and Tr saturation by iron within each group vary too significantly to make definite conclusions about the dependence of these parameters upon age. However, one 26-month old rat (in the group containing 6 rats) revealed 100% saturation. The smallest variation in Tr content and in its saturation was observed in the group of 2-month old rats. Dithionite-reduced nicotinic acid hydroxylase (NAH) from Clostridium barkeri revealed EPR signals of two different Fe-S clusters. As isolated, the enzyme did not exhibit EPR signals characteristic of Fe-S clusters, instead it showed EPR signals of Mo(V). Transformation of the Mo(V) signal after replacement of the natural mixture of Se isotopes by Se(77) suggests that Se is directly coordinated to Mo. The Se deficient enzyme lacked a Mo(V)EPR signal but it can be induced by nicotinate. The signal line shape was very different from that of Se containing enzyme, and instead, resembled the EPR signal of Mo(V) observed in xanthine oxidase. In the native seleno-enzyme, the Se atom is not directly coordinated to Fe of Fe-S clusters because substitution of Se by S did not affect EPR signals of Fe-S clusters in the reduced protein. Formate dehydrogenase revealed strong EPR signals which were attributed to the Mo(V)-pterin cofactor. Substitution of selenocysteine by cysteine led to a transformation of its line shape as was observed for NAH. Our working hypothesis now is that in both enzymes the selenium atom is directly coordinated to the Mo ion. CO dehydrogenase from Methanosarcino barkeri revealed five EPR signals derived from two different 4Fe-4S clusters, a corrinoid center, a complex of CO with the NiFe center and unidentified paramagnetic species which give rise to a broad EPR signal of 620 G width. The characteristic of CO-induced interconversion of these signals and the effect of a methylating agent could be interpreted as a direct involvement of "620" species in the enzyme catalyzed C-C bond formation.